* {
  padding: 0;
  margin: 0;
  box-sizing: border-box;

  transition-property: color, background-color, opacity, width, border-color, transform;
  transition-timing-function: var(--transition-timing-function);
  transition-duration: var(--transition-speed);

  color: var(--text-color);
}

*::selection {
  background-color: var(--secondary-color);
  color: var(--primary-color);
}

/* 为 Flexbox 添加浏览器前缀 */
.flex-center-column {
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-flex-direction: column;
  -ms-flex-direction: column;
  flex-direction: column;
  -webkit-align-items: center;
  -ms-flex-align: center;
  align-items: center;
}

.flex-center-row {
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-flex-direction: row;
  -ms-flex-direction: row;
  flex-direction: row;
  -webkit-align-items: center;
  -ms-flex-align: center;
  align-items: center;
}
